A persistent problem in the printing arts is to slow-down and shingle (i.e., partially overlap) sheets traveling at high-speed through a paper transport apparatus (e.g., printing press, or mill sheeter). It is necessary to slow sheets to a velocity, which is a fraction of the printing speed, to stack the sheets without damage. This is particularly true in the case of light-weight product, such as single sheet and four to twelve page signatures. In current high-speed paper transport apparatus, a sheet may be slowed from as much as 2900 feet per minute (fpm) to as little as 200 fpm before stacking. Such drastic changes in velocity over a very short time frame cannot be handled by traditional mechanical slow-down and shingling apparatus.
Mechanical means, such as grabbing and pinching the tail end of a sheet, have been used to slow-down sheets in slower paper transport apparatus. These mechanical means tend to mark (i.e., smear or streak) the ink on a sheet where it is grabbed or pinched when used with higher speed printing apparatus available today. For example, a rotating roller can be timed to pinch the tail of each sheet against a corresponding low-speed conveyor, which tends to "set off" or mark (i.e., smearing or streaking the ink on the sheet at the pinch location) creating waste. One solution to marking is to reduce the pinching force. However, this causes the sheet not to slow as much, potentially leading to damage to the lead edge of the sheet when it hits squaring rollers further downstream at a greater than desired velocity. It is also known that a speed limitation exists to the number of sheets which can be pinched per second without the mechanical grabbing or pinching means being able to remove the trailing edge of the now-slowed sheet from the path of the leading edge of the still rapidly moving following sheet, leading to a collision, damage to the sheets and machine shutdown.
Others have attempted to introduce a suction box to assist in solving these problems in high-speed paper transport apparatus. However, even these solutions are impractical at the very high speeds used by the most recent printing apparatus.
U.S. Pat. No. 3,178,174 to Schneider discloses a slow-down sheeter and shingling apparatus incorporating a suction box and mechanical means to push sheets into the volume affected by the suction box. A sheet traveling at a high speed exits a high-speed conveyor into an area between an upper high-speed conveyor and a lower low-speed conveyor. Beneath this sheet and on the low-speed conveyor are the previously-delivered sheets. The previously-delivered sheets are shingled and traveling at the speed of the low-speed conveyor. The mechanical means is timed to act upon the upper surface of the trailing edge of a sheet as it leaves the high-speed conveyor to push the lower surface of the trailing edge into the volume affected by the suction box located adjacent the exit of the high-speed conveyor and beneath the low-speed conveyor. The suction box pulls the trailing edge of the sheet against the low-speed conveyor. Accordingly, the sheet is slowed to the speed of the low-speed conveyor and the trailing edge of the sheet is removed from the path of the leading edge of the next sheet.
Examples of the mechanical means are a timed, rotating nozzle for compressed air, a rotating press-down bar and a stationary compressed air nozzle. At the high speeds of current printing apparatus, any mechanical means to push a sheet downward will likely limit the speed of operation of the printing apparatus. As the speed of the printing apparatus increases, the time in which the mechanical means can act to push the sheet down decreases until the allocated time to affect the sheet is inadequate. For example, once a certain speed is reached, any of the described mechanical means in Schneider will push down, and the suction box will grab, both the trailing edge of the first sheet and the leading edge of the second following sheet. The second sheet will then bind as its remainder is delivered at high speed over and past its leading edge still held by the suction box. Furthermore, if the mechanical means is then adjusted to act solely on the trailing edge of the first sheet, it may now have insufficient time to push the first sheet into the volume affected by the suction box, thus failing to remove the trailing edge of a first sheet from the leading edge of a second following sheet or causing the suction box to fail to draw the first sheet to the low-speed conveyor.
U.S. Pat. No. 5,133,542 to von Kwiatkowski also uses a suction box to assist in the slow-down of sheets traveling through a high-speed printing apparatus. Similar to Schneider, von Kwiatkowski teaches pushing the top surface of the trailing edge of a sheet toward a suction box positioned below the belts of a low-speed conveyor. In von Kwiatkowski, the leading edge of each sheet is driven upward by a stream of air as it is delivered from the high-speed conveyor driving the sheet out of the volume affected by the suction box, which is adjacent the high-speed conveyor exit. As the trailing edge of a sheet leaves the high-speed conveyor, the stream of air pushes the trailing edge into the volume affected by suction box. The trailing edge is grabbed by the vacuum box and drawn against the low-speed conveyor where it is slowed. Accordingly, multiple sheets freely float above the suction box at any given time, and as the speed of the printing apparatus increases, the likelihood of a mid-air collision that could disorient, mark or bind the sheets increases.
Finally, U.S. Pat. No. 4,436,302 to Frye et al. discloses a slow-down sheeter and shingling apparatus where slow-down rolls alone effect a slow down of each sheet, and a suction box removes the trailing edge of a sheet from the path of the leading edge of a following sheet. In Frye et al., a series of slow-down rollers positioned adjacent or above a low-speed conveyor downstream from a high-speed conveyor sequentially slow a sheet delivered from the high-speed conveyor. A suction box unassociated with the low-speed conveyor and adjacent the high-speed conveyor exit end is used solely to remove the trailing edge of a first sheet from the path of the leading edge of a second following sheet. The suction box in no way assists in slowing the sheet. Frye uses the completely mechanical slow-down rollers for this. It is known that as the speed of the printing apparatus increases, the slow-down rollers will be unable to slow a sheet without marking the sheets.